This invention relates to the field of display systems that receive input signals having multiple frame rates, more particularly to sequential color display systems that use a white light source in combination with a sequential filter such as a color wheel to produce a full color image.
Many projection display systems use a single light modulator in combination with a white light source to produce a full color image. In order to produce a full color image, the white light source is filtered sequentially to produce a primary colored light beam that changes over time. Typically, a color wheel is used to allow a series of primary colored filters to be spun through the white light beam in rapid succession. As each filter passes through the light beam, the light beam becomes a primary color beam with the active primary color determined by which portion of the color wheel is passing through the optical path.
During each primary color period, data for the appropriate color is provided to a spatial light modulator to enable the modulator to create a series of single color images. If the single color images are created in a rapid sequence, the viewer""s eye integrates the series of images to give the perception of viewing a single full-color image.
Because the data that must be written to the modulator depends on the position of the color wheel, the position of the color wheel is tightly controlled to synchronize the color wheel with the remainder of the display system. The transition period between adjacent color filtersxe2x80x94typically called a spoke periodxe2x80x94requires turning the modulator off to ensure only pure primary colored light is used to create each of the three primary colored image. Uncertainties and errors in the position or speed of the color wheel force the display system controller to lengthen the spoke periods to ensure only primary colored light is incident the modulator at the appropriate time. Unfortunately, the accumulated off time associated with the lengthened spoke periods creates a substantial drop in projector efficiency.
Another issue associated with the use of a color wheel is the need to synchronize the color wheel with the frame rate of the input video signal. This synchronization can take several seconds when a small color wheel motor is used. Small color wheel motors do not have the torque required to rapidly change the speed of the color wheel. Large color wheel motors respond to speed changes quicker, but cost more, draw more current, and take up more space in the display system.
The input video frame rate depends on the format of the input video signal. In the United States, standard television broadcast signals provide interlaced fields at a 60 Hz field ratexe2x80x94or a 30 Hz frame rate. Unless the context requires otherwise, for the purposes of this disclosure the term frame rate will be used to indicate either a frame rate or a field rate. Progressive-scanned images, which are de-interlaced, typically have a 60 Hz frame rate. Images from film typically have a 24 Hz frame rate. The frame rate of computer graphics varies between 45 Hz and 75 Hz.
Even though standard television is broadcast at 60 Hz, much of the broadcast content originated on film, which typically is shot at 24 Hz. Not only were movies that originally showed in theaters originally captured on film, much of the prime-time made for TV content was also filmed. Film is used since directors are familiar with it as a media.
After the content is captured on film, the film is converted to an interlaced 60 Hz video signal through a process called 3-2 pull-down. As shown in FIG. 1, the 3-2 pull-down process creates two video fields from the first film frame, three video fields from the second frame, two video fields from the third frame, three video fields from the fourth frame, and so on. The result of 3-2 pull-down is a series of images at the correct field and frame rate for broadcast at 60 Hz.
The 3-2 pull-down process is necessary in CRT-based display systems because the 24 Hz frame rate of the source material is too slow to avoid flicker. Some display systems, such as DMD-based displays, could display the slow frame rate image without creating flicker. While CRT-based display systems excite a phosphor once per frame, after which the phosphor gradually decays, DMD-based display systems using pulse width modulation continue to illuminate a pixel at intervals throughout the duration of the entire frame.
Unfortunately, 3-2 pull-down creates an artifact known as the 3-2 shuffle. The artifact is due to the fact that smooth motion in the original scene becomes erratic motion in the video data sequence. Typically each field of a video sequence represents the scene at a time {fraction (1/60)} of a second after the previous field and {fraction (1/60)} of a second before the next field. As shown in FIG. 1, 3-2 pull-down results in both fields of the first video frame representing the image at the same point in time. The two fields of the second video frame both represent the image at the same point in time, {fraction (1/24)} of a second after the first two fields. The third video frame is a composite of image data from video frame 2 and image data captured {fraction (1/24)} of a second late. The same is true of the fourth video frame which is a composite of information taken at the time of the last half of frame three and {fraction (1/24)} second later. Finally, the fifth video frame, like the first two, is comprised of two fields of data taken at the same point in time.
The higher frame rate that results from the 3-2 pull-down operation limits the bit depth DMD-based systems are able to display and limits the amount of time the DMD-based systems can spend on artifact mitigation techniques such as spatial-temporal multiplexing. Consequently, in order to project the best image possible, DMD-based systems detect 3-2 pull-down and decode the image signal to recreate the 24 Hz frame rate, or use a 24 Hz proscan input directly.
While the color wheel can synchronize with either a 24 Hz or 60 Hz input signal, as well as many other frequencies, switching between sources of different frame rates results in an intolerable delay while the color wheel changes speed and resynchronizes with the new input signal. What is needed is a method and system for eliminating or greatly reducing the delay necessitated by the resynchronization operation.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a display system comprising a light source for producing a beam of white light along a first light path, a filter wheel on the first light path for filtering the beam of white light, the filter wheel having at least one set of primary colored filters thereon, a motor connected to the filter wheel for spinning the filter wheel at a nominal speed, a spatial light modulator on the first light path for receiving the filtered beam of light traveling along the first path and selectively modulating the filtered beam of light traveling along the first path to form an image, and a controller receiving an input video signal at a video frame rate and providing image data decoded from the input video signal to the spatial light modulator. The input video signal has one of at least two native frame rates. The controller detects the native frame rate and converts the input video signal to the native frame rate and displays said decoded image data at the native frame rate.
According to a second embodiment, a method of operating a sequential color display system is disclosed. The method comprises the steps of spinning a color wheel at a nominal speed, receiving an input video signal, detecting a native frame rate of the input video signal, converting the input video signal to the native frame rate, displaying the input video signal at the native frame rate using the color wheel spinning at a nominal speed.
The disclosed invention provides the technical advantage of providing for a rapid change between input signals having various frame rates. A further technical advantage is the simplification of the color wheel speed controller since the color wheel is operated at a constant speed. Yet another technical advantage is the ability to perform additional image processing on signals that are detected as having a low native frame rate. By displaying an image signal at a native frame rate lower than the frame rate at which the signal is broadcast, additional processing and display time is available to use for artifact mitigation.